I think one of the AHS swim team members who is really skeptical at first about Percy's claims of being a demigod could be an interesting focus. As they start to see evidence of the supernatural world, their journey from disbelief to belief and their growth in understanding would be great to explore. They could also develop a strong friendship with Percy along the way.
A character like Annabeth could also be great if she were to be included. She could bring her wisdom and strategic thinking to the situation. Maybe she comes to help Percy with the problem at the AHS swim team. Her interactions with the swim team members, especially the ones who are into academics or are natural leaders on the team, could lead to some really engaging storylines.
A possible plot is that the AHS swim team is having a series of 'accidents' during their practices. Percy, who is passing through town, hears about it and decides to investigate. He discovers that an old enemy from his demigod days has cursed the pool. He then has to convince the swim team that the cause of their problems is not normal and get them to trust him enough to help him break the curse.
I think Percy Jackson himself would be really interesting. He has such a strong personality and his powers are unique. In a fanfiction with Descendants, he could bring his demigod skills and his sense of adventure. He might be a bit of a fish out of water in the Descendants' world, which could lead to some really funny and exciting situations. His loyalty and determination would also make him a great character to drive the story forward.
I think Mal from Descendants would be really interesting. If you combine her with the Percy Jackson universe, her skills in magic and her somewhat villainous past could create a lot of drama. She could be a bit of an outcast among the demigods at first because of her heritage, but then prove herself as a valuable ally. For example, she could use her magic to help the demigods in a battle against a new type of monster that is resistant to normal demigod powers.
Percy would be really interesting. His skills and experiences are so diverse. He could consider so many different careers like being a hero - which he already kind of is - or he could use his swimming skills for something like marine rescue. He has a great sense of adventure which could lead him to explore different career paths that are both exciting and useful.
Annabeth Chase could be an interesting character to focus on. She is highly intelligent and strategic. In a crossover, she could use her knowledge of architecture and battle strategies to help Eragon and his allies. She might study the magic in Alagaesia and find similarities or differences with the powers of the demigods. Her relationship with Percy could also be explored in a new light, especially when they are in a completely different world. How would they support each other in this new and strange environment? And how would she interact with the characters in Eragon's world?
Annabeth Chase would be an exciting character to center on. Her intelligence and strategic thinking would be a great asset in the maze. She could quickly analyze the layout of the maze and come up with plans. She might also be the one to bridge the gap between the two groups, as she's good at understanding different people and situations. Her knowledge of architecture and mazes from her own world could be applied to solving the mysteries of the Maze Runner's maze. And her relationship with Percy would add an extra layer of drama, especially when dealing with the new and dangerous situations in the crossover world.
Nico di Angelo would be an interesting choice. His dark and brooding nature along with his powers over the dead would be a contrast to the more light - hearted aspects of the Harry Potter world in some ways. He could introduce the wizards to the concept of the Underworld and the spirits. And his ability to shadow - travel could be used in some really creative ways, like getting the group in and out of dangerous situations quickly, whether it's in Hogwarts or in the magical areas related to Greek mythology.
Definitely Percy Jackson himself. His ability to control water and his overall demigod powers would make for some really cool interactions in the Halo universe. He could be a bit of an outlier among the Spartans, but his courage and quick thinking would be assets. Also, Master Chief. His stoic and badass nature would contrast well with Percy's more impulsive and humorous side. When they team up, it would be an amazing duo to write about.
Obviously, Percy Jackson himself. And there could be his swim coach, who might be either amazed or a bit freaked out by Percy's natural talent. His friends from the demigod world might also show up, like Annabeth, who could come to watch his swim meets.
Annabeth from Percy Jackson could be really interesting. She's smart and strategic. In a crossover, she could be the one to figure out how the H2O girls' powers work and how they can be used to everyone's advantage. Also, Cleo from H2O has her own charm. She comes from a different background and her insecurities combined with Annabeth's confidence would make for great character development.